This invention relates generally to optoelectronics and fiber optic communication systems and, more specifically, to a method of reducing the reflectivity in facets of optoelectronics employing waveguides, such as electro-absorption modulated semiconductor lasers (EMLs).
Semiconductor lasers, and more specifically EMLs, have become increasingly important in communication systems as they are well suited for generation and transmission of signals in fiber optic communications. Current communication systems, termed Dense Wavelength Division Multiplexed (DWDM) systems, involve channel spacing that is becoming smaller and smaller as the demand for bandwidth increases. The transmission distance between nodes and amplifiers without regeneration is also being pushed higher. These specifications have increased the need for low noise and high efficiency from the system components, such as the semiconductor lasers.
These EMLs generally include a distributed feedback (DFB) laser section 12 and an electrically isolated modulator section 20, combined as a monolithically integrated device (Ketelsen et al., IEEE Int Semiconductor Conf Sep. 1998, pp. 219; Johnson et al., IEEE Int Semiconductor Conf Sep. 1994, paper M4.7; Suzuki et al., J Lightwave Technol 1987; LT-5: 1277) (See FIG. 1). The laser portion converts electrical energy into light. The modulator portion modulates the light produced by the laser portion corresponding to an electrical signal received from a signal generator. A typical configuration, as shown in FIG. 1A, is formed on an InP substrate 12 with the grating layer 16 etched on the substrate, providing frequency-selective feedback for lasing wavelength selectivity. The fabrication of the device involves growing the layers of the DFB laser 10, isolation section 30, and the modulator section 20, using known selective area growth (SAG) processes. Narrow mesas are then etched on the wafer to form the waveguide 14, using a solution of Br2 in CH3OH with a dielectric (e.g., SiO2) mask defining the mesa pattern. A blocking structure 15 of Fe-doped InP is grown to confine the current to the active region and reduce the capacitance of the modulator. Further, a p-InP and p-InGaAs cap 17 is grown, and electrodes 18, 19 are formed on the p-side of the EML to provide electrical connection. The modulator section 20 in such known devices is collinear with the laser section 10, i.e. there is no angle between the centerlines of the waveguides of the laser and modulator sections.
The residual reflectivity of the modulator optical facet results in increased wavelength chirp and poor noise characteristics. When the absorption in the modulator changes due to the applied electrical signal, the feedback from the modulator facet 21 changes, and hence the laser threshold condition becomes modified. This results in wavelength variation and increased noise from the laser section 10. In turn, these poor characteristics degrade the transmission quality and limit the transmission performance in DWDM systems.
One prior solution to the reflectivity problem is to use anti-reflection coatings on the facets combined with a window structure to reduce the residual reflectivity to as low as xe2x88x9250 dB. This is shown in FIG. 1B, where the laser section is typically 300 xcexcm in length, the isolation section is about 140 xcexcm and the modulator is about 235 xcexcm. There is also an InP window 21 on the end of the modulator section which has an anti-reflection (HR) coating, e.g., less than 0.1%, in order to reduce the residual reflectivity. The laser section facet 11 has a high reflection (HR) coating, e.g. 85%. The overall EML chip length is 750 xcexcm.
However, significant variations in residual reflectivity occur due to the variation of the window length, the natural variation of the refractive index and the thickness of the material used for the anti-reflective coating. Any further improvements in anti-reflective coatings will likely require major process development for the coating process and for the cleaving process.
Another prior solution is the use of angled facets (Rideout et al., Electronic Letters 1990; 26: 36-37; Zah et al., Electronic Letters 1987; 23: 990-991). Angled facets, even without the use of any anti-reflective coating, have demonstrated reflectivity less than xe2x88x9240 dB. Angled facets involve tilting the waveguide relative to the cleaved facet, and have been used to obtain very low reflectivity facets. These are employed in fabricating semiconductor optical amplifiers where both facets require low reflectivity. These facets can also be applied to lasers that require anti-reflective coatings on both facets (i.e., quarter wave shifted DFB""s). However, many EMLs use a conventional DFB design for the laser portion and therefore require high reflectivity at the back facet and anti-reflective coating on the modulator facet. Thus, a simple angled facet is not appropriate for conventional DFB design, or for EMLs.
Angled facets have also been used in combination with anti-reflectivity coatings in amplifiers to achieve residual modal reflectivity to a value of 10xe2x88x924. However, since this design is anti-reflective at both facets, it is still inappropriate for use in EMLs using the conventional DFB design for the laser section.
The present invention is directed to overcoming the deficiencies of the prior art by creating an EML with ultra-low reflectivity facets that are relatively simple to manufacture, are resistant to degradation and do not require anti-reflective coatings on the facets.
In an illustrative embodiment of the invention, an EML is provided with three sections, a laser section, an isolation section, and a modulation section. The laser section produces light along a straight waveguide path. The modulation section also has a straight waveguide path, but is angled with respect to the laser section""s path. The isolation section between the other two sections includes a path that is collinear with the respective laser or modulation section at each end, but is gradually curved in the middle.